1. Field of the Invention
The present invention relates to a method for acquisition of radiological data relating to a body irradiated with x-rays and for reconstruction of structures corresponding to said body. Reconstruction of structures is aimed at the representation of images of cross-sections, or images of views, of these structures. The method is essentially concerned with the acquisition of radiological data detected by a two-dimensional detector placed opposite to the x-ray transmitter. In contrast to conventional tomodensitometers, the x-radiation in this case irradiates the body in volume and the absorption of this radiation in the body can be measured at the same time for the entire volume of the body and not only in a slice of said volume. The object of acquisitions of this type is clearly to accelerate the stage of measurement of the x-ray absorption phenomenon throughout the entire body.
2. Description of the Prior Art
This mode of acquisition is already known. For example, it was presented by Pierre Grangeat at the second image symposium of the "International Week of the Electronic Image" held at Nice in Apr., 1986. As opposed to tomodensitometry, one accordingly refers to voludensitometry, and a basic system for reconstructing the structures under study is as follows. The system comprises an x-ray point source and the radiation flux has a conical geometry. This source is placed opposite to a body to be examined. A. two-dimensional detector is placed on the other side of the body with respect to the source, substantially at right angles to a principal direction of the radiation. This two-dimensional detector is capable of acquiring radiological data which can be digitized.
By causing the acquisition unit comprising the source and the two-dimensional detector to rotate about the body, the irradiation can be reiterated and a set of acquisitions can be obtained. By means of a reconstruction algorithm, it is then possible to distribute digital data over a spatial square grid, these data being representative of a reconstruction of the structure of the body which is subjected to examination. The reconstruction algorithms employed in this type of investigation essentially involves utilization of the 3D Radon transform and its inversion formula. In other words, by reducing the acquisition stage to a few second by means of a system of this type and by employing an appropriate reconstruction algorithm, the desired result is achieved.
The reconstruction algorithms thus mentioned require that the path of travel of the x-ray source and the corresponding path of the 2D detector should be circular with respect to the body to be examined. Since the conical geometry of the radiation makes it impossible to resolve the reconstruction problem into a superposition of two-dimensional reconstructions in parallel cross-sections perpendicular to the axis of rotation of the unit, it is found necessary to develop algorithms based on approximate calculations of the 3D Radon transform and of its inversion.
The use of approximations leads to reconstruction artifacts, with the result that, when it is thus desired to visualize the reconstructed object by selecting cross-sections of this latter or by displaying it directly, these artifacts appear in the images shown. These images cannot readily be processed.
In another approach, the reconstruction is effected by simulating the reconstructed structure, by mathematically projecting the simulation of this structure in accordance with a direction of irradiation, and by comparing the image in simulated projection directly with the measurements resulting from the irradiation. In fact this comparison is made, not for a single direction of irradiation but for all directions of irradiation really carried out. Thus a projection of the simulated structure corresponds to each direction of acquisition. From this comparison is deduced a modification of the simulated model of the structure under study. In a following iteration, the modified simulated model is then projected as before, its projection is again compared with the acquisitions really made, and so on until the comparisons show that the modified simulated model is sufficiently close to the structure which has really been irradiated. These methods of reconstruction by estimation and simulation involving the use of so-called algebraic reconstruction algorithms are wholly suitable when the acquisition number is small or when the irradiations are badly distributed about the body. The number is small in particular when the examination is an angiography in which the injection of a contrast product is traumatizing and cannot be repeated too often. The irradiations are badly distributed when the geometry of the body to be studied or the means for intervention on said body do not permit correct positioning of the source-detector assembly on the entire periphery of a circle which surrounds the zone of interest of the object.
The object of the invention is to work towards the reconstruction of structures with less artifacts than in the known state of the art, this being achieved in respect of an equal acquisition number and by making use of an algebraic reconstruction algorithm. Quite simply, instead of causing the x-ray source to follow a circular path (with a corresponding path for the 2D detector), it is found preferable in accordance with the invention to distribute the orientations of the principal directions of acquisition in space so that all these orientations cannot be located in the same plane. Preferably, the acquisition number will be a minimum of three. These three orientations of the directions of irradiations will be orthogonal to each other. The avoidance orientations located in the same plane is associated with the use of an algebraic reconstruction algorithm.